The present invention relates in general to a radio receiver for receiving compatible quadrature amplitude modulation (C-QUAM) stereo radio signals, and more specifically, to detecting AM stereo signals using either of two separate stereo detection modes to minimize distortion in reproduced audio.
In commercial AM or medium-wave broadcasting, stereo stations broadcast using compatible quadrature amplitude modulation (C-QUAM) signals so that non-stereo capable receivers can still receive a compatible monophonic signal. As is known in the art, C-QUAM modulation involves phase modulating the stereo sum (L+R) and stereo difference (Lxe2x88x92R) channels in quadrature followed by multiplying the phase components by a cosine correction factor. The signal is then limited to remove any amplitude variations and is finally amplitude modulated by the monophonic (L+R) signal. At the receiver end, a non-stereo capable receiver receives a compatible signal by recovering just the final amplitude modulation. In a stereo receiver, phase information is recovered in order to detect the stereo channels. In a typical receiver, the in-phase (I) signal component and the quadrature-phase (Q) signal component are synchronously detected. An envelope detector detects the envelope of the received AM signal. The I signal and the envelope signal are compared in order to recreate the cosine correction factor. The I and Q signals are multiplied by the correction factor to reverse the modulation process previously performed at the transmitter end. The cosine-corrected I and Q signals (or the envelope signal and the Q signal) are input to a stereo decoder for decoding left and right stereo channels.
An audio output of a typical C-QUAM receiver can be extremely distorted during adverse signal reception conditions such as when over-modulation or co-channel interference exists. When these errors are introduced into the received signal, the ideal C-QUAM calculations suffer from exacerbated distortion due to phase errors.
Co-pending U.S. application Ser. No. (197-0829 ), which is incorporated herein by reference, discloses a simplified C-QUAM stereo detector which provides reduced distortion relative to normal C-QUAM detection under adverse signal reception conditions. However, this simplified detector introduces approximation errors that, although they are small for most types of broadcast material, can become noticeable for certain types of broadcast material. Thus, neither type of detector can be expected to provide the best, least distorted audio reproduction for 100% of the time.
The present invention has the advantage of selecting between stereo detection modes in order to obtain optimized audio reproduction during both good reception conditions and adverse reception conditions without having to revert to monophonic reception.
In one aspect, the present invention provides a method for reproducing left and right stereo audio signals in response to an AM stereo broadcast signal wherein a stereo sum signal and a stereo difference signal are modulated using compatible quadrature amplitude modulation (C-QUAM) including a correction factor. The broadcast signal is converted to an intermediate frequency (IF) signal. Coherent sine and cosine injection signals are generated in response to the IF signal. The sine and cosine injection signals are mixed with the IF signal to produce an in-phase demodulated (I) signal and a quadrature-phase demodulated (Q) signal, respectively. In response to at least one of the I or Q signals, either a C-QUAM mode or a pseudo-C-QUAM mode is selected for decoding the stereo sum and stereo difference signals. The C-QUAM mode includes modifying at least the Q signal according to a cosine correction factor prior to decoding the stereo sum and stereo difference signals. The pseudo-C-QUAM mode does not modify the I or Q signals according to the cosine correction factor prior to decoding the stereo sum and stereo difference signals.